In some cases it is desired to pass facsimile transmissions over a composite connection including a packet based segment. Procedures for real time transmission of facsimile messages over such composite connections are described in the ITU-T recommendation T.38 (June 1998) “Procedures for real-time Group 3 facsimile communication over IP networks”, the disclosure of which is incorporated herein by reference.
FIG. 1 shows an exemplary composite connection 10, described in the T.38 recommendation. Connection 10 passes on switched segments 13 and 17 which connect a transmitting fax machine 12 to a first gateway 14 and a second gateway 16 to a receiving fax machine 18, respectively, and a packet based segment 15 which connects gateways 14 and 16. Each of gateways 14 and 16 includes an intermediate modem and an IP interface which transfer signals between the IP network and the PSTN.
The T.38 recommendation describes standard methods for transmitting signals over composite connection 10. Generally, gateways 14 and 16 serve as relay units which pass the bits they receive between the segments of connection 10. However, some specific signals which pass on switched segments 13 and 17 are not passed along packet based segment 15. For example, when a retrain is performed on one of segments 13 or 17 the retrain is handled locally by the respective gateway and the other gateway does not have to be notified of the retrain. The procedures of the T.38 are performed in a manner that the faxes on the ends of the connection (12 and 18) are not aware of the existence of gateways 14 and 16.
The current procedures of the T.38 recommendation, however, are not suitable for use with all facsimile transmission protocols, for example transmissions in accordance with the V.34 half-duplex ITU recommendation. Generally, but not exclusively, this is because of the time constraints that the V.34 half-duplex recommendation places on the negotiation procedure, at the beginning of the facsimile connection. That is, at some specific negotiation stages, the V.34 half duplex recommendation requires that the faxes forming the connection transmit to each other signals (e.g., the MPh signals) which include information on the capabilities of the transmitting faxes. Having each of gateways 14 and 16 generate the signals locally may cause the connections on segments 13 and 17 to have different characteristics which cannot operate together. On the other hand, having gateways 14 and/or 16 wait for information from the other gateway in order to generate the signals, will usually exhaust the time allocated to the transmission of the signals, by the V.34 recommendation.
The V.34 recommendation defines a half duplex protocol for fax transmissions, which operates partially in half duplex and partially in full duplex. Generally, fax data is transmitted in a half duplex mode from a transmitting fax to a receiving fax, while during negotiation and control channel periods signals are transmitted in a full duplex mode.